Atrial tachyarrhythmia detection using selected atrial intervals

ABSTRACT

Methods and systems are directed to detecting atrial tachyarrhythmia. A plurality of A-A intervals is detected. The detected A-A intervals are selected and used to detect atrial tachyarrhythmia. Selecting A-A intervals may be based on determining that A-A intervals are qualified. Qualified A-A intervals may be determined if a duration of the particular A-A interval falls outside a predetermined duration range, for example. Qualified A-A intervals may also be determined based on events occurring between consecutively sensed atrial events of the particular A-A interval, and whether the duration of the particular A-A interval falls within the predetermined duration range, for example.

RELATED PATENT DOCUMENTS

This is a continuation of Ser. No. 12/538,294 filed Aug. 10, 2009, toissue as U.S. Pat. No. 8,041,425, which is a divisional of U.S. Ser. No.11/126,594 filed May 11, 2005, now U.S. Pat. No. 7,580,740, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to detecting atrial tachycardia.

BACKGROUND OF THE INVENTION

Proper cardiac function relies on the synchronized contractions of theheart at regular intervals. When normal cardiac rhythm is initiated atthe sinoatrial node, the heart is said to be in sinus rhythm. However,due to electrophysiologic disturbances caused by a disease process orfrom an electrical disturbance, the heart may experience irregularitiesin its coordinated contraction. In this situation, the heart is denotedto be arrhythmic. The resulting cardiac arrhythmia impairs cardiacefficiency and can be a potential life threatening event.

Cardiac arrhythmias occurring in the atria of the heart, for example,are called atrial tachyarrhythmias (ATs). ATs take many forms, includingatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid, contractions of the atria. Cardiac arrhythmiasoccurring in the ventricular region of the heart, by way of furtherexample, are called ventricular tachyarrhythmias. Ventriculartachyarrhythmias (VTs), are conditions denoted by a rapid heart beat,150 to 250 beats per minute, originating from a location within theventricular myocardium. Ventricular tachyarrhythmia can quicklydegenerate into ventricular fibrillation (VF). Ventricular fibrillationis a condition denoted by extremely rapid, non synchronous contractionsof the ventricles. This condition is fatal unless the heart is returnedto sinus rhythm within a few minutes.

Implantable cardioverter/defibrillators (ICDs) have been used as aneffective treatment for patients with serious tachyarrhythmias. ICDs areable to recognize and treat tachyarrhythmias with a variety of tieredtherapies. These tiered therapies range from providing anti-tachycardiapacing pulses or cardioversion energy for treating tachyarrhythmias tohigh energy shocks for treating atrial and/or ventricular fibrillation.To effectively deliver these treatments, the ICD must first detect thata tachyarrhythmia is occurring, after which appropriate therapy may beprovided to the heart.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for reliably andaccurately recognizing types of cardiac rhythms produced by the heart.The present invention fulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a method and system for detectingatrial tachyarrhythmia.

In accordance with a method for detecting atrial tachyarrhythmia, aplurality of A-A intervals are detected. A-A intervals from theplurality of A-A intervals are selected and used to detect atrialtachyarrhythmia.

In various embodiment of the present invention, selecting A-A intervalsincludes determining if a particular A-A interval is a qualified A-Ainterval. Qualified A-A intervals may be determined if a duration of theparticular A-A interval falls outside a predetermined duration range,for example. Qualified A-A intervals may also be determined based onevents occurring between consecutively sensed atrial events of theparticular A-A interval, and/or based on whether the duration of theparticular A-A interval falls within the predetermined duration range.Qualified A-A intervals are selected and used to detect atrialtachyarrhythmia.

In further embodiments of the invention, qualified A-A intervals areused to detect atrial tachyarrhythmia by operating a counter usingqualified A-A intervals, and detecting atrial tachyarrhythmia if thecounter reaches a predetermined value.

In another embodiment of the invention, a pacing mode switch from anatrial tracking pacing mode to a non-atrial tracking pacing mode isimplemented if atrial tachyarrhythmia is detected.

In yet another embodiment of the invention, a first atrial interval anda second atrial interval are detected, and the shorter A-A interval ofthe first and the second atrial intervals is selected.

In another embodiment of the invention, selecting the A-A intervalsincludes selecting odd numbered A-A intervals, and selecting evennumbered A-A intervals. Detecting atrial tachyarrhythmia includes usingthe odd numbered intervals to increment or decrement a first countervalue, and includes using the even numbered intervals to increment ordecrement a second counter value. Atrial tachyarrhythmia detection isbased on at least one of the first counter value and the second countervalue.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are flowcharts illustrating methods of detecting atrialtachyarrhythmia in accordance with embodiments of the invention;

FIGS. 2-3 are timing diagrams illustrating undersensing of atrialevents;

FIG. 4 illustrates atrial flutter with 3:1 ventricular pacing and atrialundersensing;

FIG. 5 illustrates atrial event undersensing during bi-ventricularpacing;

FIGS. 6A and 6B illustrate qualified and unqualified intervals,respectively;

FIGS. 7 and 8 are flowcharts illustrating methods of atrialtachyarrhythmia detection in accordance with embodiments of theinvention;

FIG. 9 illustrates a method of increasing the number of qualified A-Aintervals in accordance with embodiments of the invention;

FIG. 10 is a flowchart illustrating a method of implementing a pacingmode change from an atrial tracking mode to a non-tracking mode inaccordance with embodiments of the invention;

FIG. 11 is a flowchart illustrating a method of implementing abi-ventricular pacing therapy after a pacing mode switch from an atrialtracking mode to a non-tracking mode in accordance with embodiments ofthe invention;

FIG. 12 illustrates a method of implementing a pacing mode switch from anon-tracking mode to an atrial tracking mode in accordance withembodiments of the invention;

FIG. 13 illustrates a method in accordance with embodiments of theinvention for detecting atrial tachyarrhythmia and delivering therapy;

FIG. 14 is a flowchart illustrating a method of using qualified A-Aintervals for classifying atrial tachyarrhythmia;

FIG. 15 illustrates a method for detecting atrial tachyarrhythmia inaccordance with embodiments of the invention;

FIGS. 16A and 16B illustrate two counters used for detection of atrialtachyarrhythmia, in accordance with another embodiment of the invention;

FIG. 17 is a partial view of a cardiac rhythm management (CRM) devicethat may be used to implement atrial tachyarrhythmia detection,classification and response in accordance with embodiments of theinvention; and

FIG. 18 is an illustration of a block diagram of a cardiac rhythmmanagement (CRM) device suitable for implementing atrial tachyarrhythmiadetection, classification, and response in accordance with embodimentsof the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail hereinbelow. It is to beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

A cardiac rhythm management (CRM) device, such as an implantable cardiacpacemaker/defibrillator (PD), typically includes circuitry for sensingcardiac signals and for delivering electrical stimulation to one or moreheart chambers. Such a device may be programmed to recognize variouscardiac rhythms and provide treatment to convert, interrupt, or mitigatedangerous rhythms. A tiered approach to therapy may be implemented,wherein some rhythms are treated with a less aggressive therapy, such asanti-tachycardia pacing (ATP), other rhythms are treated with a moreaggressive therapy, such as high energy cardioversion or defibrillationshocks, and some arrhythmias are left untreated.

In addition to providing the therapies described above, the CRM may alsorespond to tachyarrhythmias by altering pacing delivered to the patient.For example, in atrial tracking modes, a fast atrial rhythm may causethe CRM device to pace the ventricle at an inappropriately high rate.Typically, pacemakers are programmed with a maximum tracking rate (MTR)that prevents the pacemaker from delivering ventricular pacing at a rateexceeding the MTR.

In some scenarios, if the atrial rate increases beyond the MTR, theventricular pacing rate may drop to the MTR so that a ventricular pulseis triggered by every other atrial event, for example. When the sinustachyarrhythmia rate is greater than the MTR, the ventricular pacing mayoccur at a N:1 ratio with respect to atrial event.

The CRM device may respond to a detected atrial tachyarrhythmia byswitching the pacing mode from an atrial tracking mode, such as DDD(R)or VDD(R) to a non-atrial tracking mode, such as DDI(R) or VDI(R). Inone implementation, if the atrial rate exceeds a trigger rate, denotedthe atrial tachyarrhythmia response (ATR) rate, then the mode switchoccurs. Mode switching limits the amount of time ventricular pacingoccurs at the maximum tracking rate. When the atrial tachyarrhythmiaepisode terminates, the pacing mode may be switched back to the atrialtracking mode.

Discriminating between different types of atrial arrhythmia allows theCRM device to select an appropriate therapy tailored for the particulartype of arrhythmia. For example, some atrial arrhythmias are responsiveto pacing therapy whereas others are more effectively treated with shocktherapy. The ability to determine the type of atrial tachyarrhythmiabefore delivering therapy may reduce the number of shocks delivered tothe patient, thus increasing the comfort of the patient and extendingthe device lifetime.

Detecting atrial tachyarrhythmia may involve determining if the atrialrate exceeds a threshold value. In one implementation, two or moreprogrammable rate zones may be used for atrial tachyarrhythmiadetection. If the atrial rate falls into a first rate zone, it isclassified as a first type of atrial arrhythmia and a first therapy maybe delivered. If the atrial rate falls into a second rate zone, theatrial arrhythmia is classified as a second type of atrial arrhythmiaand a second therapy may be delivered.

In an alternate implementation, a rate threshold may be used to detect afast atrial rate. The atrial rhythm may be further evaluated based onstability, morphology, pattern, and/or other characteristics todetermine the particular type of atrial arrhythmia.

Accurate detection of atrial tachyarrhythmia involves accurate sensingof the intrinsic atrial events of an arrhythmic episode. Sensing atrialevents occurring at a high rate is complicated due to post ventricularblanking periods that are implemented by the device followingventricular sensed or paced events. If atrial events fall within thepost ventricular blanking periods, they may not be sensed or countedtoward detection of atrial tachyarrhythmia. These unsensed atrial eventscause errors in atrial tachyarrhythmia detection, in classifying thetype of atrial tachyarrhythmia, and in pace mode switching. Undersensingof atrial events is exacerbated by bi-ventricular pacing which involvesadditional or extended blanking periods during the cardiac cycle.

Embodiments of the invention are directed to methods and systems forusing sensed atrial events for atrial tachyarrhythmia detection,classification, and response. FIG. 1A is a flowchart illustrating amethod of detecting atrial tachyarrhythmia in accordance withembodiments of the invention. Consecutive atrial events are sensed andintervals between the consecutively sensed A-A intervals are detected110. One or more of the A-A intervals are selected 120. The selected A-Aintervals are used 130 for atrial tachyarrhythmia detection.

FIG. 1B is a flowchart illustrating another method of detecting atrialtachyarrhythmia in accordance with embodiments of the invention. Inaccordance with this method, A-A intervals are detected 145 andqualified A-A intervals are selected for atrial tachyarrhythmiadetection. Determining whether or not a particular A-A interval is aqualified A-A interval includes determining 150 if the A-A intervalfalls within a predetermined duration range. If the A-A interval fallsoutside 150 the predetermined duration range, then the A-A interval is165 a qualified interval.

If the A-A interval falls within 150 the predetermined duration range,the timing of events occurring within the A-A interval are evaluated155. Whether or not the A-A interval is 160 a qualified A-A interval isbased on the timing of events falling within the A-A interval. QualifiedA-A intervals are used 170 in atrial tachyarrhythmia detection. In someimplementations, a relationship between the timing of an event occurringwithin an A-A interval with respect to a point within the A-A intervalmay be used to determine if an interval is qualified. For example, anA-A interval may be determined to be qualified if the midpoint of theA-A interval does not fall within a blanking period occurring within theA-A interval.

FIGS. 2-3 are timing diagrams illustrating undersensing of atrialevents. The timing diagram of FIG. 2 illustrates atrial flutter (AFL)resulting in 2:1 ventricular pacing with no undersensing of atrialevents. Following the first atrial event, A1, a pacing cycle isinitiated. An AV delay is initiated and the ventricle is paced, Vp, atthe end of the AV delay. Following the ventricular pacing pulse, a crosschamber blanking period, PVAB, and a cross chamber refractory period,PVARP, are initiated. A second atrial event, A2, occurs during thePVARP, but after expiration of the PVAB. Thus, A2 is sensed, but is notused to initiate a pacing cycle.

The next pacing cycle is initiated by the third atrial event, A3, and issimilar to the pacing cycle initiated by A1. A ventricular pacing pulseoccurs after expiration of the AV delay. Cross chamber blanking andrefractory periods, PVAB and PVARP, follow the ventricular pace. Thenext atrial event, A4, is sensed following expiration of PVAB but beforeexpiration of PVARP. Because A4 is sensed during PVARP, A4 is not usedto initiate a new pacing cycle. The pacing illustrated in FIG. 2 isrepresentative of 2:1 behavior, wherein every other atrial event causesa pacing cycle to be initiated and the ventricle is paced atapproximately one-half the atrial rate.

FIG. 3 is a timing diagram illustrating AFL producing 2:1 ventricularpacing with undersensing of atrial events. As illustrated in FIG. 3,every other atrial event is unsensed and every other atrial eventinitiates a pacing cycle. Following the first atrial event, A1, a pacingcycle is initiated. The ventricle is paced, Vp, at the end of the AVdelay. Following Vp, a cross chamber blanking period, PVAB, and a crosschamber refractory period, PVARP, are initiated. A second atrial event,A2, occurs during PVAB. Thus, A2 is not sensed and is not used toinitiate the next pacing cycle.

The second pacing cycle is initiated by the third atrial event, A3, andis similar to the pacing cycle initiated by A1. A Vp occurs afterexpiration of the AV delay. Cross chamber blanking and refractoryperiods, PVAB and PVARP, follow the ventricular pace. The next atrialevent, A4, is sensed during PVAB and is not sensed.

FIG. 4 illustrates AFL with 3:1 ventricular pacing and atrialundersensing. In this situation, every other atrial event is sensed andone out of three atrial events initiates a pacing cycle. Following thefirst atrial event, A1, a pacing cycle is initiated. A pacing pulse, Vp,is delivered at the end of the AV delay. Following Vp, a cross chamberblanking period, PVAB, and a cross chamber refractory period, PVARP, areinitiated. A second atrial event, A2, occurs during PVAB. Thus, A2 isnot sensed and is not used to initiate the next pacing cycle. The nextatrial event A3 is sensed during PVARP of the first pacing cycle. A3 issensed, but is not used to start a pacing cycle.

The next pacing cycle is initiated by the fourth atrial event, A4, andis similar to the pacing cycle initiated by A1. A Vp is delivered afterexpiration of the AV delay. Cross chamber blanking and refractoryperiods, PVAB and PVARP, follow the ventricular pace. A5 is sensedduring PVAB and is not sensed nor used to start a pacing cycle. A6 issensed during PVARP and is sensed, but is not used to start a pacingcycle.

FIG. 5 illustrates atrial event undersensing during bi-ventricularpacing. A sensed atrial event that does not fall within a PVARP is usedto initiate an AV delay for a cardiac pacing cycle. If the rightventricle is paced, RVp, following the AV delay, the opposite ventricleis paced, LVp, following an interventricular delay (IVD). A PVAB periodand a PVARP are initiated by the left ventricular pace. Thus, whenbi-ventricular pacing is delivered, post ventricular blanking in theatrium is increased by the interventricular delay (IVD) which may have aduration of up to about 100 ms.

Detection of atrial tachyarrhythmia involves counting the number of A-Aintervals that fall into one or more atrial tachyarrhythmia rate zones.Undersensing of atrial events, as illustrated in the examples of FIGS.3-5, may cause failure or delays in satisfying rate zone detectioncounters used in detection of atrial tachyarrhythmia. Further, the longA-A intervals caused by atrial undersensing may cause errors in theclassification of types of atrial tachyarrhythmia, e.g., atrialfibrillation vs. atrial flutter. Further, undersensed atrial events maycause delays in implementation of atrial tachyarrhythmia therapy orinappropriate mode switching. For example, atrial undersensing may causedelays in mode switching or oscillations in switching back and forthbetween tracking mode and non-tracking mode. As described below inaccordance with various exemplary embodiments, the problems associatedwith atrial undersensing may be reduced by using qualified intervals foratrial tachyarrhythmia detection and/or classification.

In one implementation of atrial tachyarrhythmia detection, the deviceuses one or more counters to determine the number of fast atrial eventsoccurring within a rate zone detection window. For example, a rate zonedetection window may be satisfied if x out of y, e.g., about 32 out ofabout 40, of the most recent A-A intervals are short A-A intervals,corresponding to a high atrial rate. After the detection window issatisfied, then it will remain satisfied if a predetermined number e.g.,about 24 out of about 40, of the most recent A-A intervals are shortatrial intervals.

The device compares each detected A-A interval to a predeterminedinterval value, denoted the atrial tachyarrhythmia response interval(ATRI), associated with a fast atrial rate. The rate zone counter isincremented if a detected A-A interval is shorter than the ATRI and isdecremented if a detected A-A interval is longer than the ATRI. When thecounter reaches a predetermined value, then an atrial tachyarrhythmiaepisode is detected. Additional short A-A intervals may be used toconfirm the atrial tachyarrhythmia episode. During the atrialtachyarrhythmia episode, the counter is incremented by short intervalsand decremented by long intervals as before. If the counter valuereaches zero, the atrial tachyarrhythmia episode is determined to haveterminated.

The use of only qualified atrial intervals to increment or decrement theatrial detection counter may allow reliable detection of atrialarrhythmias. For example, using qualified atrial intervals, thedetection window may be satisfied if about 24 out of about 30 or ifabout 16 out of about 20 are shorter than the ATRI.

In accordance with one embodiment of the invention, the following A-Aintervals are considered to be qualified atrial intervals:

Qualified A-A Interval Criteria Set 1

-   -   1) A-A<ATRI, even though interrupted by a ventricular pace;    -   2) A-A>N×ATRI, where N is about 2; and    -   3) ATRI<A-A<N×ATRI and the A-A interval is not interrupted by a        ventricular pace.

The above qualified A-A interval criteria set may be used if the deviceis not able to discern the timing of a PVAB that falls within the A-Ainterval. In some implementations, the device may be able to determinethe timing of the PVAB with respect to a PVAB falling within the A-Ainterval. If so, then the criteria for qualified A-A intervals may bemodified as follows:

Qualified A-A Interval Criteria Set 2

-   -   1) A-A<ATRI, even though interrupted by a ventricular pace;    -   2) A-A>N×ATRI, where N is about 2;    -   3) ATRI<A-A<N×ATRI and the A-A interval is not interrupted by a        ventricular pace    -   4) ATRI<A-A<N×ATRI and the midpoint of A-A interval does not        occur during PVAB.

FIGS. 6A and 6B illustrate qualified and unqualified intervals,respectively, according to criterion 4 above. FIG. 6A, illustratesqualified A-A intervals having midpoints that do not fall within PVAB.FIG. 6B illustrates unqualified A-A intervals that have midpoints thatfall within PVAB.

In yet another embodiment, the following criteria may be used toidentify qualified A-A intervals:

Qualified A-A Interval Criteria Set 3

-   -   1) A-A<ATRI, even though interrupted by a ventricular pace;    -   2) A-A>N×ATRI, where N is about 2;    -   3) ATRI<A-A<N×ATRI and the A-A interval is not interrupted by a        ventricular pace; and    -   4) ATRI<A-A<N×ATRI and both of the following:        -   the midpoint of A-A interval does not occur during PVAB;        -   A to Vp greater than ATRI or Vp to A greater than ATRI+PVAB.

The criteria sets described above are provided as example criteria sets.Criteria sets for identifying qualified A-A intervals may includeadditional criteria or alternative criteria to those presented in theexamples above.

FIG. 7 is a flowchart illustrating a method of atrial tachyarrhythmiadetection in accordance with embodiments of the invention. The methodillustrated in FIG. 7 corresponds to Criteria Set 1, above. Atrialevents are sensed and A-A intervals between consecutively sensed atrialevents are detected 705. If the length of the atrial interval is 710between the ATRI and twice the ATRI and the A-A interval is notinterrupted 715 by a ventricular pace, then the A-A interval is 720 aqualified long A-A interval and is used 725 to decrement the ATRcounter. If the length of the atrial interval is 710 between the ATRIand twice the ATRI and the A-A interval is not interrupted 715 by aventricular pace, then the A-A interval is not a qualified A-A interval.The unqualified A-A interval is not used for atrial tachyarrhythmiadetection and the next A-A interval is detected 705.

If the A-A interval is less than or equal to 730 the ATRI, then the A-Ainterval is 745 a qualified short interval and it is used to increment750 the ATR counter. If the A-A interval is greater than or equal to 740twice the ATRI, then the A-A interval is 720 a qualified long intervaland is used to decrement 725 the ATR counter. If the ATR counter valuereaches 760 a predetermined value, then atrial tachyarrhythmia isdetected 765.

FIG. 8 is a flowchart illustrating a method of atrial tachyarrhythmiadetection in accordance with embodiments of the invention. The methodillustrated in FIG. 8 corresponds to Criteria Set 2, above. Atrialevents are sensed and A-A intervals between consecutively sensed atrialevents are detected 805. If the length of the A-A interval is 810between the ATRI and twice the ATRI and the A-A interval does notinclude 812 a ventricular pace, then the A-A interval is a qualifiedlong interval 820 and is used to decrement the ATR counter.

If the A-A interval is 810 between the ATRI and twice the ATRI and theA-A interval and the A-A interval includes 812 a ventricular pace theA-A interval is a qualified long interval 820 if the midpoint of the A-Ainterval does not fall 815 within a blanking period. The A-A intervaland is used 825 to decrement the ATR counter. If the length of theatrial interval is 810 between the ATRI and twice the ATRI and themidpoint of the A-A interval falls within 815 a blanking period, theninterval is not used for tachyarrhythmia detection and the systemdetects 805 the next interval.

If the A-A interval is less than or equal to 830 the ATRI, then the A-Ainterval is 845 a qualified short interval and it is used to increment850 the ATR counter. If the A-A interval is greater than or equal to 840twice the ATRI, then the A-A interval is a qualified long interval 820and is used to decrement 825 the ATR counter. If the ATR counter valuereaches 860 a predetermined value, then atrial tachyarrhythmia isdetected 865.

In some scenarios, the number of qualified A-A intervals occurringwithin a time period may not be sufficient for to detect arrhythmia,classify the arrhythmia and/or determine an appropriate response to thearrhythmia. In these situations, additional processes may be implementedto mitigate undersensing of atrial events to increase the number ofqualified A-A intervals, or to otherwise enhance arrhythmia detection,classification, and/or response processes. FIG. 9 illustrates a methodof increasing the number of qualified A-A intervals in accordance withembodiments of the invention. Atrial events are sensed and A-A intervalsare detected 905. Qualified atrial intervals 910 are used to incrementor decrement 915 an atrial tachyarrhythmia detection counter asdescribed in connection with the examples above. When the counter valuereaches 920 a predetermined value, then atrial tachyarrhythmia isdetected 925.

If a predetermined number of sequential non-qualified A-A intervals, forexample, about 3 non-qualified A-A intervals, are detected 930,processes to reduce atrial undersensing may be initiated. In oneembodiment, reducing atrial undersensing involves decreasing 935 themaximum pacing rate, for example by about 10 bpm. Decreasing the maximumpacing rate may increase the number of qualified intervals available fortachyarrhythmia detection.

As previously discussed, if atrial tachyarrhythmia is detected, thedevice may switch the pacing mode from an atrial tracking mode to anon-tracking mode. Pacing continues in the non-tracking mode for aperiod of time or so long as the atrial tachyarrhythmia is present.After the atrial rate drops, the device may switch back to the atrialtracking pacing mode.

FIG. 10 is a flowchart illustrating a method of implementing a pacingmode change from an atrial tracking mode to a non-tracking mode inaccordance with embodiments of the invention. A-A intervals are detected1005 and a determination is made as to whether the A-A intervals arequalified 1010. In instances where three consecutive unqualified A-Aintervals occur 1045, then bi-ventricular pacing is disabled 1050 for anumber of beats, e.g., about one beat. For a qualified interval 1010,and if bi-ventricular trigger pacing is not disabled 1015, eachqualified A-A interval increments or decrements 1025 the atrialtachyarrhythmia counter by one. For a qualified interval 1010 withbi-ventricular trigger pacing disabled 1015, each qualified A-A intervalincrements or decrements 1020 the atrial tachyarrhythmia detectioncounter by a predetermined number, e.g., about 2. When the atrialtachyarrhythmia counter reaches 1030 a predetermined value, then a modeswitch occurs 1040 from the atrial tracking mode to the non-trackingmode.

FIG. 11 is a flowchart illustrating a method of implementing abi-ventricular pacing therapy after a pacing mode switch 1110 from anatrial tracking mode to a non-tracking mode in accordance withembodiments of the invention. Several ventricular beats, e.g., abouteight beats are monitored 1120 to determine the consistency of theconduction pattern. If 1130 the rhythm is not 2:1 AFL, bi-ventricularpacing is enabled. If 1130 the rhythm is 2:1 AFL, prior to enablingbi-ventricular trigger pacing, the system determines if bi-ventriculartrigger pacing will cause 1140 undersensing of atrial events. Ifundersensed atrial events would not 1140 occur during bi-ventriculartrigger pacing, then bi-ventricular trigger pacing is enabled 1160. Ifundersensed atrial events would 1140 occur during bi-ventricular triggerpacing, then bi-ventricular trigger pacing may be disabled 1150 untilafter atrial therapy is delivered.

As previously discussed, detection of an atrial tachyarrhythmia maycause a pacing mode switch from an atrial tracking mode to anon-tracking mode. When the atrial tachyarrhythmia subsides, then thepacing mode may be switched back to tracking mode. FIG. 12 illustrates amethod of implementing a pacing mode switch from a non-tracking mode toan atrial tracking mode in accordance with embodiments of the invention.Following a pacing mode change 1210 to a non-tracking mode, A-Aintervals are detected 1220. An atrial tachyarrhythmia counter isupdated 1230 (incremented or decremented) using qualified A-A intervals.Updating the detection counter involves incrementing the counter if ashort qualified A-A interval is detected and decrementing the counter ifa long qualified A-A interval is detected. When the counter valuereaches 1240 zero, a sufficient number of long qualified A-A intervalshave occurred within the detection window to determine that the atrialtachyarrhythmia has subsided. The pacing mode is switched 1260 from thenon-tracking mode to an atrial tracking mode.

If the current rhythm is 1245 undersensed 2:1 AFL and bi-ventricularpacing is being delivered, then a rhythm change is monitored 1250 whilethe counter non-zero. If a rhythm change to normal sinus rhythm isdetected 1255 then the pacing mode is switched 1260 from thenon-tracking mode to atrial tracking.

In some circumstances, a lack of qualified A-A intervals in atachyarrhythmia episode may cause delays in atrial tachyarrhythmiadetection and therapy delivery. FIG. 13 illustrates a method inaccordance with embodiments of the invention for detecting atrialtachyarrhythmia and delivering therapy. In accordance with this method,A-A intervals are detected 1310 and qualified A-A intervals 1320 areused to update 1330 the atrial tachyarrhythmia detection counter. If thecounter reaches 1340 a predetermined value, indicating the presence ofatrial tachyarrhythmia, then atrial tachyarrhythmia is detected 1350 andtherapy is delivered 1360.

If a sufficient number of qualified A-A intervals are not detected 1320within a predetermined time period 1370, for example, about 30 seconds,then further processing 1380, 1390 occurs to determine if therapy shouldbe delivered. The rhythm is evaluated 1380 and if the current rhythm is2:1 undersensed AFL, then the device checks to see if a rhythm changehas occurred 1390. If a rhythm change is not detected 1390, then atrialtachyarrhythmia therapy is delivered 1360. If a rhythm change isdetected 1390, then the device continues to detect A-A intervals 1310for atrial tachyarrhythmia detection.

In some implementations, the system may classify the type of arrhythmiathat is occurring. Classifying the type of atrial tachyarrhythmia may beuseful in selecting an appropriate therapy to treat the arrhythmia. Forexample, some atrial arrhythmias, such as atrial flutter, are paceterminable, whereas other atrial arrhythmias, such as atrialfibrillation, are more effectively treated using shocks.

In some embodiments, qualified A-A intervals are used to classify thetype of atrial arrhythmia that is occurring. FIG. 14 is a flowchartillustrating a method of using qualified A-A intervals for classifyingatrial tachyarrhythmia. According to this embodiment, an A-A interval isdetected 1410 and the system determines 1420 if the detected A-Ainterval is a qualified A-A interval. If a sufficient number ofqualified A-A intervals have been acquired 1430, then the qualified A-Aintervals are used to evaluate the rhythm. The rhythm is evaluated 1440and the type of atrial rhythm is classified using the qualified A-Aintervals. An appropriate therapy may be delivered 1480 based on theatrial rhythm classification.

If the detected A-A interval is 1420 not qualified, then the systemdetermines if a sufficient number of qualified A-A intervals aredetected 1445 within a predetermined time period, for example, about 30seconds. Where the time period has not expired 1445, A-A intervalscontinue to be detected. Where the time period has expired, adetermination 1450 is made as to whether the atrial rhythm is 2:1 AFLwith undersensed atrial events. If 2:1 AFL with atrial eventundersensing is determined 1450 to be present, then the systemdetermines 1460 if rhythm change has occurred. If a rhythm change is notdetected, then AFL therapy is delivered 1470. If a rhythm change isdetected, or if the current rhythm is determined 1450 not to be 2:1 AFLwith atrial event undersensing, then classification of the rhythm isdelayed until a sufficient number of qualified A-A intervals areacquired.

Processes described above involve selecting qualified A-A intervals foratrial tachyarrhythmia detection. In accordance with some embodiments ofthe invention, the selection of A-A intervals may not be evaluated todetermine if the A-A intervals are consistent with qualifying criteria,such as the exemplary qualified A-A interval criteria sets describedabove. A method for detecting atrial tachyarrhythmia in accordance withembodiments of the invention, is illustrated in the flowchart of FIG.15. According to this method two sequential A-A intervals are detectedand the shortest of the two A-A intervals is used to update the atrialtachyarrhythmia detection counter. This method provides more sensitiveand stable atrial tachyarrhythmia detection and response.

As illustrated in FIG. 15, first and second A-A intervals are detected1510, 1520. The duration of the first A-A interval is compared 1530 tothe duration of the second A-A interval. If the first A-A interval isshorter than 1540 the second A-A interval, then the first A-A intervalis selected 1545 for use in atrial tachyarrhythmia detection. If thesecond A-A interval is shorter than 1540 the first A-A interval, thenthe second A-A interval is selected 1550 for use in atrialtachyarrhythmia detection.

The duration of the selected interval is compared to the duration of adetection interval. If the selected interval is longer than 1555 theduration of detection interval, the selected A-A interval is a long A-Ainterval. Long A-A intervals are used to decrement 1560 the atrialtachyarrhythmia counter. If the selected interval is shorter than 1555the duration of detection interval, then the selected interval is ashort A-A interval. Short A-A intervals are used to increment 1565 theatrial tachyarrhythmia counter. After incrementing 1565 or decrementing1560 the atrial tachyarrhythmia counter, a determination 1570 aboutwhether the atrial tachyarrhythmia window is satisfied 1570, e.g., if xout of y A-A intervals are short A-A intervals. When satisfied, atrialtachyarrhythmia is detected 1575. Otherwise, the process re-starts witha first A-A interval detection 1510.

In accordance with another embodiment of the invention, FIGS. 16A and16B illustrate two counters used for detection of atrialtachyarrhythmia. Odd numbered A-A intervals, e.g., 1^(st), 3^(rd),5^(th) etc. detected A-A intervals, are used to operate a first counter.Even numbered A-A intervals, e.g., 2^(nd), 4^(th), 6^(th) etc., detectedA-A intervals, are used to operate the second counter. If either thefirst or the second counters reach a predetermined count, atrialtachyarrhythmia is detected.

According to the method as illustrated in the flowchart of FIGS. 16A and16B, an A-A interval is detected 1605 and the system ascertains whetherthe A-A interval is an odd or even numbered interval in the sequence. Ifthe A-A interval is 1610 an odd numbered interval in the sequence, thenthe odd numbered A-A interval is selected 1615 to operate the oddsequence atrial tachyarrhythmia counter.

The duration of the selected interval is compared to the duration of adetection interval. If the selected interval is longer than 1620 theduration interval, the selected A-A interval is a long A-A interval.Long A-A intervals are used to decrement 1625 the odd sequence atrialtachyarrhythmia counter. If the selected interval is shorter than 1620the duration interval, then the selected interval is a short A-Ainterval. Short A-A intervals are used to increment 1630 the oddsequence atrial tachyarrhythmia counter. Based on the atrialtachyarrhythmia counter, if the atrial tachyarrhythmia window issatisfied 1635 for odd sequence A-A intervals, e.g., if x out of y oddsequence A-A intervals are short A-A intervals, then atrialtachyarrhythmia is detected 1640. If the atrial tachyarrhythmia count isnot satisfied, then A-A intervals are detected 1605.

If the A-A interval is 1610 an even numbered interval in the sequence,then the even numbered A-A interval is selected 1650 (FIG. 16B) tooperate the even sequence atrial tachyarrhythmia counter.

The duration of the selected interval is compared to the duration of adetection interval. If the selected interval is longer than 1655 theduration interval, the selected A-A interval is a long A-A interval.Long A-A intervals are used to decrement 1660 the even sequence atrialtachyarrhythmia counter. If the selected A-A interval is shorter than1655 the duration interval, then the selected interval is a short A-Ainterval. Short A-A intervals are used to increment 1665 the evensequence atrial tachyarrhythmia counter. If the atrial tachyarrhythmiawindow is satisfied 1635, e.g., if x out of y even sequence A-Aintervals are short A-A intervals, then atrial tachyarrhythmia isdetected 1640.

FIG. 17 is a partial view of a cardiac rhythm management (CRM) devicethat may be used to implement atrial tachyarrhythmia detection,classification and response in accordance with embodiments of theinvention. Methods of the invention may be implemented in a variety ofimplantable or patient-external cardiac therapeutic and/or diagnosticdevices including, for example, pacemakers, defibrillators,cardioverters, bi-ventricular pacemakers, and/or cardiacresynchronization devices, among others. The CRM device illustrated inFIG. 17 includes an implantable housing 1700 containing circuitryelectrically coupled to an intracardiac lead system 1702. Portions ofthe implantable housing may be configured as a can electrode 1709. Thehousing 1700 and the intracardiac lead system 1702 is implanted in ahuman body with portions of the intracardiac lead system 1702 insertedinto a heart 1701. The intracardiac lead system 1702 is used to detectelectric cardiac signals produced by the heart 1701 and to provideelectrical energy to the heart 1701 under predetermined conditions totreat cardiac arrhythmias.

The intracardiac lead system 1702 includes one or more electrodes usedfor pacing, sensing, and/or defibrillation. In the particular embodimentshown in FIG. 17, the intracardiac lead system 1702 includes a rightventricular lead system 1704, a right atrial lead system 1705, and aleft ventricular lead system 1706. In one embodiment, the rightventricular lead system 1704 is configured as an integrated bipolarpace/shock lead.

The right ventricular lead system 1704 includes an SVC-coil 1716, anRV-coil 1714, and an RV-tip electrode 1712. The RV-coil 1714, which mayalternatively be configured as a separate defibrillation coil and anRV-ring electrode 1711, is spaced apart from the RV-tip electrode 1712,which is a pacing electrode for the right ventricle.

The right atrial lead system 1705 includes a RA-tip electrode 1756 andan RA-ring electrode 1754. The RA-tip 1756 and RA-ring 1754 electrodesmay provide pacing pulses to the right atrium of the heart and may alsobe used to detect cardiac signals from the right atrium. In oneconfiguration, the right atrial lead system 1705 is configured as aJ-lead.

In the configuration of FIG. 17, portions of the intracardiac leadsystem 1702 are shown positioned within the heart 1701, with the rightventricular lead system 1704 extending through the right atrium and intothe right ventricle. Typical locations for placement of the RV-tipelectrode 1712 are at the right ventricular (RV) apex or the RV outflowtract.

In particular, the RV-tip electrode 1712 and RV-coil electrode 1714 arepositioned at appropriate locations within the right ventricle. TheSVC-coil 1716 is positioned at an appropriate location within a majorvein leading to the right atrium chamber of the heart 1701. The RV-coil1714 and SVC-coil 1716 depicted in FIG. 17 are defibrillationelectrodes.

The left ventricular lead system 1706 is advanced through the superiorvena cava (SVC), the right atrium 1720, the ostium of the coronarysinus, and the coronary sinus 1750. The left ventricular lead system1706 is guided through the coronary sinus 1750 to a coronary vein of theleft ventricle 1724. This vein is used as an access pathway for leads toreach the surfaces of the left atrium and the left ventricle which arenot directly accessible from the right side of the heart. Lead placementfor the left ventricular lead system may be achieved via subclavian veinaccess and a preformed guiding catheter for insertion of the leftventricular (LV) electrodes 1713 and 1717 adjacent the left ventricle.In one configuration, the left ventricular lead system 1706 isimplemented as a single-pass lead.

An LV distal electrode 1713, and an LV proximal electrode 1717 may bepositioned adjacent to the left ventricle. The LV proximal electrode1717 is spaced apart from the LV distal electrode, 1713 which is apacing electrode for the left ventricle. The LV distal 1713 and LVproximal 1717 electrodes may also be used for sensing the leftventricle.

The lead configurations illustrated in FIG. 17 represent oneillustrative example. Additional lead/electrode configurations mayinclude additional and/or alternative intracardiac electrodes and/orepicardial electrodes. For example, in one configuration, anextracardiac lead may be used to position epicardial electrodes adjacentthe left atrium for delivering electrical stimulation to the left atriumand/or sensing electrical activity of the left atrium.

Referring now to FIG. 18, there is shown a block diagram of a cardiacrhythm management (CRM) device 1800 suitable for implementing atrialtachyarrhythmia detection, classification, and response in accordancewith embodiments of the invention. FIG. 18 shows a CRM device 1800divided into functional blocks. It is understood by those skilled in theart that there exist many possible configurations in which thesefunctional blocks can be arranged. The example depicted in FIG. 18 isone possible functional arrangement. Various functions of the CRM device1800 may be accomplished by hardware, software, or a combination ofhardware and software.

The CRM device 1800 includes components for sensing cardiac signals froma heart and delivering therapy, e.g., pacing pulses orcardioversion/defibrillation shocks, to the heart. The circuitry of theCRM device 1800 may be encased and hermetically sealed in a housing 1801suitable for implanting in a human body. Power to the circuitry issupplied by an electrochemical battery power supply 1880 that isenclosed within the housing 1801. A connector block with lead terminals(not shown) is additionally attached to housing 1801 to allow for thephysical and electrical attachment of the intracardiac lead systemconductors to the encased circuitry of the CRM device 1800.

In one embodiment, the CRM device 1800 includes programmablemicroprocessor-based circuitry, including control circuitry 1820, amemory circuit 1870, sensing circuitry 1831, 1832, 1835, 1836, and apulse generator 1841. Components of the CRM device 1800 cooperativelyperform operations involving atrial tachyarrhythmia detection accordingto the approaches of the present invention. The control circuitry 1820is responsible for arrhythmia detection, classification, and therapycontrol. The control circuitry 1820 may encompass various functionalcomponents, for example, an arrhythmia detection/classification circuit1821, an arrhythmia counter 1823 and a therapy control unit 1822. Thearrhythmia detection/classification circuit 1821 performs processesdescribed above including selecting and using A-A intervals for atrialtachyarrhythmia detection and/or classification. The arrhythmia counter1823 is used to count selected intervals for arrhythmia detection and/orclassification. The arrhythmia detection/classification circuit 1821 maybe used in connection with determining an appropriate response to atrialtachyarrhythmia, e.g., pace mode switching or therapy to treat theatrial tachyarrhythmia.

The memory circuit 1870 may store program instructions used to implementthe functions of the CRM device 1800 as well as data acquired by the CRMdevice 300. For example, the memory circuit 1870 may store historicalrecords of sensed cardiac signals, including arrhythmic episodes, and/orinformation about therapy delivered to the patient. The memory circuit1870 may also store morphology templates representative of cardiac beatsassociated with various types of cardiac rhythms.

The historical data stored in the memory 1870 may be used for variouspurposes, including diagnosis of patient diseases or disorders. Analysisof the historical data may be used to adjust the operations of the CRMdevice 1800. Data stored in the memory 370 may be transmitted to anexternal programmer unit 1890 or other computing device, such as anadvanced patient management system as needed or desired.

Telemetry circuitry 1860 allows the CRM device 1800 to communicate withan external programmer unit 1890 and/or other remote devices. In oneembodiment, the telemetry circuitry 1860 and the external programmerunit 1890 use a wire loop antenna and a radio frequency telemetric linkto receive and transmit signals. In this manner, programming commandsand data may be transferred between the CRM device 1800 and the externalprogrammer 1890 after implant.

The CRM device 1800 may function as a pacemaker and/or a defibrillator.As a pacemaker, the CRM device 1800 delivers a series of electricalstimulations to the heart to regulate heart rhythm. Therapy controlcircuitry 1822 controls the delivery of pacing pulses to treat variousarrhythmic conditions of the heart, for example. In various embodiments,the CRM device 1800 may deliver pacing pulses to one or more of theright atrium, left atrium, right ventricle and the left ventricle. Theheart may be paced to treat bradycardia, or to synchronize and/orcoordinate contractions of the right and left ventricles.

For example, right ventricular pacing may be implemented using unipolaror bipolar configurations. Unipolar RV pacing involves, for example,pacing pulses delivered between the RV-tip 1712 to can 1709 electrodes.Bipolar pacing involves, for example, delivery of pacing pulses betweenthe RV-tip 1712 to RV-coil 1714 electrodes. If an RV-ring electrode ispresent, bipolar pacing may be accomplished by delivering the pacingpulses to the RV-tip 1712 and RV-ring 1711 electrodes.

Left ventricular pacing may be implemented using unipolar or bipolarconfigurations. Unipolar LV pacing may include, for example, pacingpulses delivered between the LV distal electrode 1713 and the can 1709.Alternatively, bipolar LV pacing may be accomplished by delivering thepacing pulses using the LV distal electrode 1713 and the LV proximalelectrode 1717.

Similarly, unipolar (RA-tip electrode 1756 to can electrode 1709) atrialpacing or bipolar (RA-tip electrode 1756 to RA-ring electrode 1754)atrial pacing may be provided by the CRM device 1800.

The CRM device 1800 may also provide tachyarrhythmia therapy. Forexample, tachyarrhythmia therapy may be provided in the form ofanti-tachycardia pacing (ATP) pulses delivered to an atrium or aventricle. The ATP pulses may involve a series of timed paces ofprogrammable width and amplitude that are implemented to interrupt atachyarrhythmia episode. The ATP therapy may involve, for example, burstpacing at about 25 Hz to about 50 Hz. In various implementations, thepace-to-pace interval may have a variable or constant length. ATPtherapy may be delivered to treat atrial flutter, for example. Therapyfor atrial fibrillation may involve cardioversion shocks to the heartthat may be initiated automatically or by the patient. Life threateningarrhythmias, such as ventricular fibrillation may be treated by one ormore defibrillation shocks to the heart to terminate the fibrillation.

In the embodiment depicted in FIG. 18, electrodes RA-tip 1756, RA-ring1754, RV-tip 1712, RV-ring 1711, RV-coil 1714, SVC coil 1716, LV distalelectrode 1713, LV proximal electrode 1717, and can 1709 are coupledthrough a switching matrix 1810 to various sensing circuits 1831, 1832,1835, 1836. A right atrial sensing channel circuit 1831 serves to senseand amplify electrical signals from the right atrium of the heart. Forexample, bipolar sensing in the right atrium may be implemented bysensing signals developed between the RA-tip 1756 and RA-ring 1754electrodes. The switch matrix 1810 may be operated to couple the RA-tip1756 and RA-ring 1754 electrodes to the RA sensing channel circuit 1831to effect bipolar sensing of right atrial signals. Alternatively,unipolar right atrial sensing may be accomplished by operating theswitch matrix 1810 to couple the RA-tip 1756 and can 1709 electrodes tothe RA sensing channel circuit 1831.

Cardiac signals sensed through the use of the RV-tip electrode 1712 andRV-coil 1714 or RV-ring electrode 1711 are right ventricular (RV)near-field signals and are referred to as RV rate channel signalsherein. Bipolar rate channel sensing may be accomplished by operatingthe switch matrix 1810 to couple the RV-tip electrode 1712 and theRV-coil 1714 electrode or the RV-ring electrode 1711 through the RV ratechannel sensing circuitry 1835. The rate channel signal may be detected,for example, as a voltage developed between the RV-tip electrode 1712and the RV-coil 1714 electrode or the RV-ring electrode 1711. The RVrate channel sensing circuitry 1835 serves to sense and amplify the RVrate channel signal.

Unipolar RV sensing may be implemented, for example, by coupling theRV-tip 1712 and can 1709 electrodes to the RV rate channel sensingcircuitry 1835. In this configuration, the rate channel signal isdetected as a voltage developed between the RV-tip 1712 to can 1709sensing vector.

The RV lead system may also include an RV-ring electrode 1711 used forbipolar pacing and sensing. If an RV-ring electrode is included in thelead system, bipolar sensing may be accomplished by sensing a voltagedeveloped between the RV-tip 1712 and RV-ring 1711 electrodes.

Far-field signals, such as cardiac signals sensed through use of one ofthe defibrillation coils or electrodes 1714, 1716 and the can 1709, orusing both of the defibrillation coils or electrodes 1714, 1716, arereferred to as morphology or shock channel signals herein. The shockchannel signal may be detected as a voltage developed between theRV-coil 1714 to the can electrode 209, the RV-coil 1714 to the SVC-coil1716, or the RV-coil 1714 to the can electrode 1709 shorted to theSVC-coil 1716. The switch matrix 1810 is operated to couple the desiredshock channel sensing vector, e.g., RV-coil to can, to the rightventricular shock channel sensing circuitry 1832. The RV shock channelsensing circuitry 1832 serves to sense and amplify the shock channelsignal.

The outputs of the switching matrix 1810 may also be operated to coupleselected combinations of the electrodes to LV sensing channel circuitry1836 for sensing electrical activity of the left ventricle. Bipolar leftventricular sensing may be accomplished by operating the switch matrix1810 to couple the LV-distal 1713 and the LV proximal electrodes 1717through the LV channel sensing circuitry 1836. In this configuration,the LV signal is detected as a voltage developed between the LV proximaland LV distal electrodes.

Unipolar LV sensing may be implemented, for example, by coupling the LVdistal 1713 and can 1709 electrodes to the LV sensing circuitry 1736. Inthis configuration, the LV signal is detected as a voltage developedbetween the RV-tip 1712 to can 1709 sensing vector.

The CRM device 1800 may incorporate one or more metabolic sensors 1845for sensing the activity and/or hemodynamic need of the patient.Rate-adaptive pacemakers typically utilize metabolic sensors to adaptthe pacing rate to match the patient's hemodynamic need. A rate-adaptivepacing system may use an activity or respiration sensor to determine anappropriate pacing rate. Patient activity may be sensed, for example,using an accelerometer disposed within the housing of the pulsegenerator. Transthoracic impedance, which may be measured, for example,via the intracardiac electrodes, may be used to determine respirationrate. Sensor information from the metabolic sensor is used to adjust thepacing rate to support the patient's hemodynamic need. If the sensorsindicate the patient's activity and/or respiration rate is high, thenthe patient's pacing rate is increased to correspond to the level ofactivity or rate of respiration.

It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments discussed hereinabovewithout departing from the scope of the present invention. Accordingly,the scope of the present invention should not be limited by theparticular embodiments described above, but should be defined only bythe claims set forth below and equivalents thereof.

1. A method for detecting atrial tachyarrhythmia, comprising: sensingatrial events; detecting A-A intervals between the atrialdepolarizations in a sequence of atrial depolarizations; identifying atleast first and second A-A intervals of the sequence; selecting at leastone A-A interval of the sequence for use in tachyarrhythmia detectionbased on information about the first A-A interval relative to the secondA-A interval, wherein selecting the at least one A-A interval comprisesat least one of comparing a duration of the first A-A interval to aduration of the second A-A interval, and determining whether the atleast one A-A interval is an odd interval or an even interval in thesequence; operating at least one counter using the selected A-Ainterval; and detecting tachyarrhythmia based on an output of thecounter.
 2. The method of claim 1, wherein selecting the at least oneA-A interval further comprises: selecting a shortest of the first andthe second A-A intervals; comparing the duration of the selected A-Ainterval to a detection interval; and operating the counter comprisesincrementing or decrementing the counter based on a result of comparingthe duration of the selected A-A interval to the detection interval. 3.The method of claim 2, wherein operating the counter comprises:decrementing the counter if the duration of the selected A-A interval islonger than the detection interval, and; incrementing the counter if theduration of the selected A-A interval is shorter than the detectioninterval.
 4. The method of claim 2, wherein selecting the at least oneA-A interval comprises selecting a predetermined number of the shortestA-A intervals.
 5. The method of claim 1, wherein the informationcomprises odd/even sequence information.
 6. The method of claim 1,wherein selecting the at least one A-A interval further comprises:selecting an odd numbered interval, and selecting an even numberedinterval; operating the at least one counter comprises: using the oddnumbered interval to increment or decrement a first counter, and usingthe even numbered interval to increment or decrement a second counter;and detecting tachyarrhythmia comprises detecting tachyarrhythmia basedon at least one of an output of the first counter and an output of thesecond counter.
 7. A cardiac device, comprising: sensing circuitryconfigured to sense atrial depolarizations; a therapy controllerconfigured to control cardiac pacing therapy; and an atrialtachyarrhythmia detector configured to detect A-A intervals betweenconsecutive atrial depolarizations, identify A-A intervals in a sequenceselect A-A intervals of the sequence for use in atrial tachyarrhythmiadetection based on at least one of: information about a first of a pairof sequential A-A intervals relative to a second of the pair ofsequential A-A intervals, wherein the information comprises at least oneof a comparison of a duration of a first A-A interval to a duration of asecond A-A interval, and odd/even sequence information, and informationabout events occurring between consecutively sensed atrialdepolarizations of the A-A intervals, operate at least one counter usingthe selected A-A intervals, and detect tachyarrhythmia based on anoutput of the counter.
 8. The cardiac device of claim 7, wherein theatrial tachyarrhythmia detector is configured to select the A-Aintervals based on comparison of the A-A intervals to a durationinterval.
 9. The cardiac device of claim 7, wherein the informationabout events occurring between consecutively sensed atrialdepolarizations comprises timing of a blanking period between theconsecutively sensed atrial depolarizations.
 10. The cardiac device ofclaim 7, wherein the information about events occurring betweenconsecutively sensed atrial depolarizations includes information aboutwhether the A-A intervals are interrupted by ventricular events.
 11. Thecardiac device of claim 7, wherein the atrial tachyarrhythmia detectoris configured to select a shortest of the first and the second A-Aintervals and to compare the duration of the selected A-A interval to adetection interval, and increment or decrement the counter based oncomparison of the duration of the selected A-A interval to the detectioninterval.
 12. The cardiac device of claim 11, wherein the atrialtachyarrhythmia detector is configured to decrement the counter if theduration of the selected A-A interval is longer than the detectioninterval and increment the counter if the duration of the selected A-Ainterval is shorter than the detection interval.
 13. The cardiac deviceof claim 11, wherein the atrial tachyarrhythmia detector is configuredto select a predetermined number of the shortest A-A intervals.
 14. Thecardiac device of claim 7, wherein the atrial tachyarrhythmia detectoris configured to select an odd numbered interval and select an evennumbered interval and to use the odd numbered interval to increment ordecrement a first counter value and to use the even numbered interval toincrement or decrement a second counter value and to detecttachyarrhythmia based on at least one of the first counter value and thesecond counter value.